The invention concerns a switching arrangement for receiving direct current signals in telegraphic or data transmission systems.
As is generally known, distortions occur during the transmission of DC signals, which hinder the reception of the individual signal elements making up such signals. As is well known, a transmission circuit comprises not only the transmission line proper, but also the sending and receiving circuits terminating the line at both ends. Thus, two distortion-causing influences can essentially be distinguished.
One type of distortion to which DC signals are subject is caused by the fact that each line has capacitive and resistive components. Charging and discharging or charging and recharging of the line takes place with every change in the signal element through interruption or polarity reversal of the line caused by the transmitter. The time constant of the resistance-capacitance (RC) network formed by the transmission circuit leads to rounding in varying degrees of the signal waveforms. This occurs primarily in the case of on/off keying.
A second type of distortion is caused by the influence of inductive elements where, most significantly, the influence of the inductances on the sending and/or receiving circuits cannot fully be removed. The result is that such a transmission circuit, including transmission line and receiving and/or sending circuit, constitutes an oscillatory configuration. This means that a change on the line, for example, one caused by interruption or a short-circuit polarity reversal produces oscillations that initially die out slowly. Since, as a rule, the receivers are to be polarity-independent, the subharmonies of such oscillations can also be interpreted if conventional keying is employed. This may produce the result that the received signals have a sequence of brief current pulses and no-current intervals after every signal-element shift.
Thus, both the first type of distortion, also called RC distortion, and the second type, also called RLC distortion, require additional correcting means at the receiving end. Heretofore, to avoid RC distortions, the threshold value was set such that undershooting or exceeding of a threshold value by a signal-element shift is relayed with mimimum distortion. However, since the influences causing the RC distortions may vary from line to line, it is often necessary to adjust the threshold for each individual line. Despite the fact that these circuits contain a vast number of elements, it is not always possible to distinguish pulses caused by disturbances from true signal elements. Heretofore, special circuits, such as a start-signal test circuit, were necessary to eliminate such distortions.
Influences caused by subharmonics can likewise be avoided by properly adjusting the threshold value in the receiver circuit, but the optimum adjustment of the threshold value with respect to the reception of signals distorted by RLC influences causes a deterioration with respect to the reception of signals distorted by RC influences.
It is, therefore, an object of the invention to provide a receiver circuit by which distorted received DC signals can be interpreted substantially without distortion, while avoiding the disadvantages mentioned hereinabove.